1. Field of the Invention
The present invention relates in general to a piston operable to force frictional coupling elements of a clutch or brake provided in an automatic transmission, against each other to place the clutch or brake in its engaged state.
2. Discussion of the Related Art
A known automatic transmission includes clutches and brakes each of which is provided with frictional coupling elements in the form of a plurality of friction plates, so that the automatic transmission is automatically shifted by selectively engaging and releasing the clutches and brakes. The frictional coupling elements are forced against each other by a piston, to place each clutch or brake in its engaged state. JP-9-32919A discloses an example of such a piston, which consists of two members, namely, a first piston member forming a cylindrical portion of the piston, and a second piston member forming a bottom portion of the piston. In this piston disclosed in JP-9-32919A, the first piston member (cylindrical portion) has cutouts, while the second piston member (bottom portion) has engaging portions fitted in the cutouts, so that the first and second piston members are fixed to each other into the piston, with a retainer ring.
Where the piston is constituted by the first piston member forming the cylindrical portion and the second piston member forming the bottom portion, which are fixed to each other with a retainer ring or the like, a small gap inevitably exists in the axial direction of the piston, between the first and second piston members fixed together with the retainer ring, due to axial dimensional errors within a predetermined tolerance, so that the first and second piston members are undesirably movable relative to each other in the axial direction of the piston, in the presence of the above-indicated gap. Accordingly, the first piston member tends to have an axial rattling movement, giving rise to a risk of unnecessary abutting contact of the first piston member with the outermost one of the frictional coupling elements, which may cause unnecessary mutual frictional contact of the frictional coupling elements, leading to so-called “dragging” of the clutch or brake, and consequent deterioration of durability of the piston and the frictional coupling elements, and an increase in power loss of the automatic transmission due to the dragging.